This invention relates to microwave directional coupling circuits, and more particularly, to coupling circuits embodied in a microstrip configuration.
Microstrip directional couplers provide acceptable signal coupling levels, but exhibit relatively poor characteristics in terms of isolation and directivity. The poor qualities of directivity and isolation arise due to the microstrip configuration, that is having a single ground plane on one side only. One technique used in the past to overcome this problem has been to insert a stripline coupler in a microstrip circuit. This technique has the obvious disadvantages of placing a portion of stripline circuitry into a microstrip structure.
Other methods in the past have been to add either lumped or distributed capacitances by interdigitizing portions of the signal line and the coupling line. Some of these structures are illustrated in an article by Allen Podell appearing in EDN magazine of Jan. 5, 1974 beginning at page 56. These structures however are designed for operation at a single frequency or over a narrow single range of frequencies, and require print and etch techniques for their construction. Their use is primarily in established designs and they are not suitable for breadboard designs. The print and etch techniques also have the associated problems of over etching or under etching thereby providing additional problems in manufacture.
Therefore, a microstrip directional coupler which is simple in construction and can be readily breadboarded, easily tuned over a wide range of frequencies, and provides high isolation across a wide bandwidth is highly desirable.